I wrote a letter this week to Delia Moffatt, the main character in my new story, from her dead mother. The mother does not appear in the story, having died when Delia was five, eight years before the story begins. But I wrote the letter as a means for me to understand the complicated relationships between some of my characters.
I am sorry that I can't be there to see the delightful young girl you are today. How relieved I was that Grandma moved back in with you and Daddy after I left. You have Esme too, remember -- my lifelong, closest friend. She means it when she says she will do anything for you. Don't let Daddy be too proud to ask her . . . "
The letter goes on, but you get the idea. This little exercise has helped me to get more of the back story straight.
It's not necessary for the reader to know where Delia's and Sam's parents came from, but I need to be aware of these details as I write. The letter will not appear in the book, and I will use only a small part of the back story, but in order for me to tell the story, I have to have a sense of what happened before the opening scene.